1. Field of the Invention
This invention relates to radiation-induced crosslinking of polymers using diacetylenes as sensitizing agents.
2. Brief Description of the Prior Art
The crosslinking of polymers is a technique well known in the art, which is normally achieved either by chemical treatment with a free radical reagent or treatment with actinic radiation and results in crosslinked polymers useful as fabricated products. Crosslinking by chemical treatment is relatively inexpensive compared to crosslinking induced by radiation, but radiation-induced crosslinking has a significant advantage in one respect in that one can, if desired, produce higher concentrations of crosslinked polymer in localized regions in the fabricated product. For instance, in a rubber stamp, it is desired to have a hard surface as the image to be stamped, but it is also desirable to have a softer spongy undersurface for flexibility when impressing the stamp upon a surface. Another example is the sole of a shoe where it is desired to have a hard surface for firmness and traction, but a softer spongy undersurface for wearing comfort.
Crosslinked polymers typically show higher dimensional and thermal stability, tensile strength and solvent resistivity than uncrosslinked polymers. These polymers find a number of important industrial uses as in heat shrinkable tubing which is used for a variety of applications, including electrical insulation and also for the packaging of various materials. When a polymer is subjected to actinic radiation, such as gamma radiation, a number of chemical changes occur like crosslinking, chain degradation, and the formation of radicals and unsaturated centers. The term crosslinking refers to the union of high polymer molecules by a system involving primary chemical bonds. These bonds are usually carbon to carbon bonds which are formed between the polymer chains resulting from the breaking of carbon to hydrogen bonds in the individual polymer molecules. Crosslinking has the effect of binding polymer chains into a single network so that it becomes functionally a single molecule, i.e., it changes the thermoplastic properties of a polymer to thermosetting ones.
The cost of radiation-induced crosslinking of polymers can be reduced by incorporating crosslinking accelerators, which are also called crosslinking agents or sensitizers, into the polymer. Radiation-induced crosslinking is carried out with actinic radiation, which is usually gamma radiation produced by a radioactive cobalt source, and is relatively expensive. The use of a sensitizer or accelerator agent, such as the diacetylenes of this invention, acts to accelerate the rate at which crosslinking occurs and thus, reduces the amount of radiation needed to produce the same quantity of crosslinking, and thus substantially lowers the cost of the process. Sensitizers are also useful in promoting crosslinking reactions in some polymers, e.g. isobutylene, in which chain degradation would otherwise result upon irradiation in the absence of a sensitizer or crosslinking agent. Sensitizers for use in radiation-induced crosslinking are well known in the art.
Acetylene is a known sensitizer for radiation induced crosslinking and is exemplified in Kagiya, et al., Janapese Pat. No. 74 05,608 (1974), where it is used as a sensitizer for the crosslinking of methyl methacrylate-vinyl chloride graft copolymer.
Propiolic acid is another acetylene derivative used as a sensitizer and is exemplified in kagitani, et al., Japanese Pat. No. 74 23,581 (1974), in which polyethylene and polypropylene is crosslinked with ionizing radiation in the presence of propiolic acid.
Dipropargyl phthalate is also a known sensitizer and is exemplified in Okamoto et al., Japanese Pat. No. 74 44,577 (1974) where dipropargyl phthalate is used for sensitizing the crosslinking of polyolefins.
Propargyl alcohol is also a known crosslinking agent as exemplified in French Pat. No. 2187-816 (1974), which illustrates the use of this alcohol as a sensitizer in the crosslinking of ethylenic polymers.
Another sensitizing agent containing terminal acetylene groups is exemplified in West German Patent DT 2308-576 (1974) where various acetylene derivatives of s-triazine are used for the crosslinking of poly-alpha olefins.
The above sensitizing agents, however, all contain terminal acetylene groups, which could, when incorporated into the final fabricated product, render the product sensitive to the action of acids and alkalies. This results in discoloration and marring of the surface of the fabricated article due to the chemical interaction between the terminal acetylene groups and acids and alkalies.
Rasch, et al. in U.S. Pat. No. 3,822,134 (1974) disclose the use of diacetylenes as compounds capable of forming colored images upon exposure to radiation on surfaces, after vapor deposition of the diacetylene upon the surface. However, the use of diacetylenes as sensitizing agents for the crosslinking of polymers is not suggested.
Non-terminal diacetylenes of the formula [Y--O--(CH.sub.2).sub.n --C.tbd.C--.sub.2 in which n is 1 and Y is hydrogen and p-toluene-sulfonyl, and non-terminal diacetylenes of the formula [Q--NHCO--O--(CH.sub.2).sub.n --C.tbd.C--.sub.2 in which n is 1 and Q is phenyl and substituted phenyl, and wherein n is 2 or an integer greater than 2 and Q is alkyl, phenyl and substituted phenyl, have been found to be unsuitable as accelerating agents during the radiation-induced crosslinking of polymers.
However, a new class of diacetylene sensitizing agents useful in radiation-induced crosslinking has been discovered which does not contain terminal acetylene groups with their attendant disadvantages.